Satellites are being used increasingly for providing communication links between stations of different locations throughout the world. Initially, such links were provided for use at sea where, given the availability of power from ocean-going vessels, direct communications to satellites could be achieved. Recently, this approach has been extended to mobile units, powered from local batteries, using relatively low orbiting satellites.
In order for low orbiting satellites to remain in orbit, it is not possible for them to orbit at the rotational speed of the earth and thereby remain geostationary. As a result, there is a relative velocity between the ground stations and the orbiting satellites and procedures must be implemented in order to establish a communication link, at any one time, with a satellite that is within range. Thus, such an approach could be compared to terrestrial cellular systems, in which the ground stations remain stationary with a relative velocity being created by a roaming mobile.
In terrestrial situations in which a mobile roams between transmission cells, it is necessary to provide complex handover routines as a mobile moves from cell to cell, as is known in the art. However, the relative speed of the mobiles, compared to the transmission frequencies, is negligible therefore the movement of the mobile does not affect the characteristics of the signal itself and, from a signal analysis point of view, the mobiles may be treated as being effectively stationary.
In satellite systems the problem of roaming from one satellite position to another is also present and again complex procedures must be put in place to ensure that a communication link can be handed over from one system to another while a call is taking place. However, another problem is introduced in that the velocity of the orbiting satellites is such that the transmitted signals will experience Doppler effects. Thus, transmission frequencies will appear higher as the satellite is moving towards a ground station and, similarly, the transmission frequency will appear lower as the satellite is moving away from said ground station. Thus, frequency channels from one satellite may impinge upon channels from a neighbouring satellite unless a very large spacing is provided between adjacent channels or groups of channels from respective satellites. Furthermore, most frequency channels convey a plurality of digital signals as a time division multiplex and frequency shifting may therefore disrupt channel allocation of the time division multiplex. Furthermore, timing offsets will also occur due to the position of the satellite. Thus, before a connection may be established for the transmission of actual data, it is necessary to identify the nature of the data so as to identify the extent to which the frequency of a message has been modified by Doppler effects and the extent to which the start of the message has been shifted in phase so as to locate the message within the correct time division multiplex channel.